The direct backlight illuminating device has been more and more widely used. Generally, the direct backlight illuminating device uses the LED module which can be designed as chain, plate or puzzle module. However, it is difficult to install the chain module in the light box of a big size because the lighting modules of the chain module should be connected together to form the chain module and to be installed in the light box, which will certainly prolong the installing time and increase the difficulty in installation. The plate module inevitably has the defect of having a high structure weight. And as to the type of puzzle module in the prior art, it also has the defect of having a high structure weight. Moreover, the modules of the above types can hardly meet the requirement of double-side light box, because the LED lighting components in the modules of the above types merely can be arranged on one side of the module. In order to meet the requirement of double-side lighting, at least two groups of modules should be simultaneously arranged to emit light to both sides, respectively, which will necessarily increase the weight of the whole system. A plurality of arcuate rods assembled to form a lamp fence is disclosed in an earlier patent application US application US2003/0026095 A1. However, the lamp fence is used for decoration but not for an illuminating device. Moreover, the assembled fence shape is not suitable for an illuminating device due to the uneven distribution of light.